Some people tried it and got sued for patent infringement (ASML vs XTAL)[1]. ASML patents all EUV-related technology, so you'd have to come up with a brand new way of generating EUV plasma at high energies at constant rate. Even ASML had to buy an American company Cymer to get a working prototype. Then, because EUV radiation doesn't pass through air or standard lenses, you'd have to come up with another optic column system. If you want to go 5nm and below, you need anamorphic lenses/mirrors with almost atomic precision [2]. ASML gets these columns and lenses/mirrors from Zeiss, but probably has patents on their practical application.

The sheer size of the supply chain you need to create to make a working machine is massive. It's one of the reasons why ASML got ahead of the Japanese companies that tried to do everything in-house.[3][4]

It took 17+ years and €6 billion in R&D to get EUV working [5] (as such EUV litography became feasible in the 90s), so ASML's leading position on the market is justified. They have to recoup all the costs, so they will make sure to keep the market leadership through all legal means.

My guess is that if someone would come up with a more efficient EUV plasma generation or better optic systems, ASML would acquire the company and take over the technology.

Regarding the negotiating power, some companies like GlobalFoundries couldn't afford ASML's EUV machines, so they parked their EUV plans [6]. It's only big players like TSMC, Samsung and Intel that can afford this tech and all these companies know their worth and prices. ASML needs money to pay for the R&D and can't afford to lose it's biggest customers that are also buying their non-bleeding edge technologies (DUV litography).

I don't know the business side of things, but it's definitely going to be interesting to see what happens with competition in the EUV litography in the upcoming 10+ years.

EUV litography and silicon-based chips have their physical limits, so there might be other ways to continue improvements.[7]

[1] https://www.reuters.com/article/us-asml-holding-xtal-court-i...

[2] https://semiengineering.com/extending-euv-to-2nm-and-beyond/

[3] https://ideas.repec.org/p/eti/dpaper/05007.html

[4] https://youtu.be/SB8qIO6Ti_M?t=673

[5] https://www.asml.com/en/products/euv-lithography-systems

[6] https://www.anandtech.com/show/13277/globalfoundries-stops-a...

[7] https://semiengineering.com/making-chips-at-3nm-and-beyond/